上一篇中只介绍AppDelegate中applicationDidFinishLaunching()函数，这个函数是程序运行的关键，在CCApplicationProtocol中声明纯虚函数，在CCApplication中进行调用。在CCApplicationProtocol中与applicationDidFinishLaunching()类似的纯虚函数还有两个，分别是：applicationDidEnterBackground()和applicationWillEnterForeground()，在CCApplicationProtocol中声明如下：

class CC_DLL CCApplicationProtocol
{
public:
	/**
	@brief    Implement CCDirector and CCScene init code here.
	@return true    Initialize success,app continue.
	@return false   Initialize Failed,app terminate.
	*/
	virtual bool applicationDidFinishLaunching() = 0;

	/**
	@brief  The function be called when the application enter background
	@param  the pointer of the application
	*/
	virtual void applicationDidEnterBackground() = 0;

	/**
	@brief  The function be called when the application enter foreground
	@param  the pointer of the application
	*/
	virtual void applicationWillEnterForeground() = 0;
}@H_404_3@在AppDelegate对其中类成员函数进行覆写，AppDelegate.h中的代码如下： 
 
 
#ifndef  _APP_DELEGATE_H_
#define  _APP_DELEGATE_H_

#include "cocos2d.h"

/**
@brief    The cocos2d Application.

The reason for implement as private inheritance is to hide some interface call by CCDirector.
*/
class  AppDelegate : private cocos2d::CCApplication
{
public:
    AppDelegate();
    virtual ~AppDelegate();

    /**
    @brief    Implement CCDirector and CCScene init code here.
    @return true    Initialize success,app continue.
    @return false   Initialize Failed,app terminate.
    */
    virtual bool applicationDidFinishLaunching();

    /**
    @brief  The function be called when the application enter background
    @param  the pointer of the application
    */
    virtual void applicationDidEnterBackground();

    /**
    @brief  The function be called when the application enter foreground
    @param  the pointer of the application
    */
    virtual void applicationWillEnterForeground();
};

#endif // _APP_DELEGATE_H_
@H_404_3@  由官方的注释就可知道applicationDidEnterBackground()和applicationWillEnterForeground()的作用，applicationDidEnterBackground()函数是游戏程序进入后台运行时候调用的，applicationWillEnterForeground()函数是游戏程序从后台恢复到前台运行时候调用的。 

   再来看看AppDelegate.cpp中的内容，代码如下： 
 
 
#include "AppDelegate.h"
USING_NS_CC;
AppDelegate::AppDelegate() {

}
AppDelegate::~AppDelegate() 
{
}
bool AppDelegate::applicationDidFinishLaunching() {
    // initialize director
    CCDirector* pDirector = CCDirector::sharedDirector();
    CCEGLView* pEGLView = CCEGLView::sharedOpenGLView();
    pDirector->setOpenGLView(pEGLView);	
    // turn on display FPS
    pDirector->setDisplayStats(true);
    // set FPS. the default value is 1.0/60 if you don't call this
    pDirector->setAnimationInterval(1.0 / 60);
	CCScene *pScene = HelloWorld::scene();
    // run
    pDirector->runWithScene(pScene);
    return true;
}

// This function will be called when the app is inactive. When comes a phone call,it's be invoked too
void AppDelegate::applicationDidEnterBackground() {
    CCDirector::sharedDirector()->stopAnimation();

    // if you use SimpleAudioEngine,it must be pause
    // SimpleAudioEngine::sharedEngine()->pauseBackgroundMusic();
}

// this function will be called when the app is active again
void AppDelegate::applicationWillEnterForeground() {
    CCDirector::sharedDirector()->startAnimation();

    // if you use SimpleAudioEngine,it must resume here
    // SimpleAudioEngine::sharedEngine()->resumeBackgroundMusic();
}
@H_404_3@ 
AppDelegate的构造函数与析构函数是空的，构造函数运行时候通过执行父类构造函数初始化CCApplication单例对象，之后没有其他功能。
 
CCDirector* pDirector = CCDirector::sharedDirector();得到了一个CCDirector单例对象并把指针赋值给pDirector 。
 CCEGLView* pEGLView = CCEGLView::sharedOpenGLView();Cocos2d-x对OpenGL进行了封装，Cocos2d-x对OpenGL进行了初始化。
 pDirector->setOpenGLView(pEGLView); 设置了场景的窗口。
 
 
pDirector->setDisplayStats(true);启用FPS显示，设置是否显示游戏的帧数等调试信息。
 
 
pDirector->setAnimationInterval(1.0 / 60);设置游戏的帧率，即每秒刷新画面次数，这里是60帧每秒。
 
 
其中重点关注以下两句：
 
 
CCScene *pScene = HelloWorld::scene();
    // run
    pDirector->runWithScene(pScene);@H_404_3@ 

 上面的第一句：CCScene *pScene = HelloWorld::scene();创建了一个场景，这个场景就是HelloWorld，然后把这个场景指针赋值给pScene，之后通过pDirector->runWithScene(pScene);把这个HelloWorld场景运行起来。我们需要知道的是pDirector是一个CCDirector单例对象的指针，CCDirector是控制游戏中各种元素的类。 

 
 
 总结一下：CCApplicationProtocol中定义了运行时用到的几个接口，前台运行、进入后台运行、返回前台运行三个接口；CCApplication中包装了跨平台的实现；AppDelegate 则具体实现了CCApplicationProtocol定义的各个接口。
 原文链接：https://www.f2er.com/cocos2dx/342903.html